ANKARA (Reuters) - Iranian opposition leader Mehdi Karroubi called off his hunger strike on Thursday after authorities accepted one of his demands and withdrew security agents from his home where he has been held since 2011, his website said.

Karroubi was hospitalised earlier in the day shortly after launching a hunger strike to demand a public trial and withdrawal of security men from his residence, which is under 24-hour surveillance by the powerful Revolutionary Guards.

Karroubi's son, Mohammad Taghi Karroubi, confirmed the report, saying via Twitter: "My father has called off his hunger strike after ... the withdrawal of the agents from his house."

"Officials have also promised to take measures regarding my father's demand for a public trial," Karroubi's son wrote.

Opposition leaders Karroubi, Mirhossein Mousavi and his wife Zahra Rahnavard have been confined to their homes for six-and-a-half years after calling for rallies in solidarity with pro-democracy uprisings then shaking Arab countries.

They have never been put on trial or publicly charged. Both Karroubi, 80, and Mousavi, 75, suffer from ailments partly associated with their age. Karroubi has been hospitalised twice in recent weeks and underwent heart surgery.

Karroubi's website, Sahamnews, had quoted the Shi'ite cleric's wife, Fatemeh Karroubi, as saying that he would refuse "to eat or drink until his demands are met".

Karroubi and Mousavi ran in what became a disputed 2009 election that returned hardline President Mahmoud Ahmadinejad to power and triggered mass protests ultimately crushed by the elite Revolutionary Guards and its affiliated Basij militia.

Dozens of political activists, lawmakers, journalists and artists have urged pragmatist President Hassan Rouhani to fulfil a campaign promise to get the opposition leaders freed.

Some insiders say that Iran's top authority, Supreme Leader Ayatollah Ali Khamenei, opposes their release and a public trial.

"He does not expect a fair trial but wants it to be public and would respect the verdict," Karroubi's wife told Sahamnews on Wednesday.

"Such a level of surveillance has never been seen before or after the (1979 Islamic) revolution ... He wants the authorities to announce when they will hold a public trial."

The Center for Human Rights in Iran (CHRI) called on Wednesday for the immediate release of "ailing political leaders" in the Islamic Republic.

"Karroubi's life is in danger and the state, which has detained him without trial, is responsible for whatever happens to him while he is in its custody," CHRI executive director Hadi Ghaemi said in a statement.